This invention relates to security threads for paper-based value documents such as currency and banknote papers, and more particularly to a device for sensing the security thread and for determining the authenticity and denomination of the document therefrom.
There exists a number of different approaches in the prior art for verifying the authenticity of paper-based value documents, such as currency and banknote papers, bank checks, stock certificates, etc. These or other methods may also be used to verify a characteristic of the document, such as the denomination of the currency paper. In this way different features of the same general class of documents may be identified. However, verifying the denomination of the currency paper may also be interpreted to be a verification of the authenticity of the document as well.
All of the known verification approaches rely on the detection and/or measurement of specific physical properties or patterns associated with the documents. Usually, the feature to be detected is deliberately added to the document during document manufacture as part of a document recognition system or an anti-counterfeit document verification system. The device used to ascertain the type of security feature added to the document, as well as to distinguish between various characteristics of the document (as indicated by certain features designed into the type of security feature), is usually designed in conjunction with the physical characteristics of the security feature. This is to provide optimum functionality in document verification.
Common approaches include the usage of magnetic ink printed at predetermined locations and in predetermined patterns on a surface of the paper. Another approach is to embed into the currency paper, either partially or entirely, a plastic security thread substrate coated with predetermined patterns of conductive and/or magnetic materials. The detector is then designed to sense the type of material and, to a limited extent, the spatial distribution of the material on the thread substrate.
More specifically, prior uses of magnetic materials in the field of document security have strictly involved relatively "hard" (i.e., high magnetic coercivity) magnetic materials. The magnetic material may be formed as part of the ink printed on a surface of the document, may be introduced into the surface of the document in some other form, or may be coated on the plastic substrate of a security thread embedded in the document.
Detection of these relatively hard magnetic materials (and, thus, verification of the authenticity of the document and/or some characteristic thereof) is typically carried out by exposing the material to a magnetic field and then detecting the remanent magnetization. The magnetic field may be applied to the magnetic material either at the time of document manufacture, or by the detection system itself just prior to "reading" or sensing the remanent magnetization; e.g., during a commercial sales transaction or during bank sorting of the currency paper. Examples of relatively hard magnetic materials utilized in the aforementioned applications include magnetic powders, such as ferrites, or thin sheets or ribbons of crystalline magnetic material, such as nickel. (See U.S. Pat. No. 4,183,989.) Patterns of magnetization may be written to the materials, and the patterns can be read with reading heads. The reading heads are capable of reading either direct current (D.C.) magnetization (e.g., Hall-effect sensing), or may utilize a time-varying magnetic field generated by movement of the bill past the read head. In either case, only the net remanent magnetization is measured. This approach requires use of high-strength magnetic fields for pre-magnetization and sensitive read heads for detection. A limitation is that detection of the magnetic material must take place at close proximity (much less than 1 millimeter spacing between the read head and the magnetic material). Examples of this "hard" magnetic material approach to document verification are given in EP 0295229, WO 92/08226, EP 0319524, EP 0204574, EP 0428779, WO 91/04549, GB 2130414, WO 91/10902, EP 0413534 and U.S. Pat. 3,870,629.
In contrast to "hard" magnetic materials and their usage in document security, it is known to use relatively "soft" magnetic materials (i.e., low magnetic coercivity) in the field of electronic article surveillance (e.g., anti-theft detection of items in a retail store environment). Compared to hard magnetic materials, the soft magnetic materials are easily magnetizable from a distance by a relatively weak applied magnetic field. A typical application includes the retail article having a "tag" or "marker" comprised of the soft (e.g., ferromagnetic) material attached thereto. If the article is legitimately purchased, the clerk at the retail store either removes the article or causes a change in the marker's magnetic characteristics. However, if the article is attempted to be stolen, an interrogating magnetic field applied to the exit area of the retail store strikes the marker, which then gives off or emits characteristic, recognizable signals. These signals may be utilized to sound an alarm to alert store personnel as to the attempted occurrence of a theft.
These prior art surveillance applications have involved the detection of a tagged object at essentially unconstrained position or orientation within a relatively large volume of space. A soft magnetic material comprising the marker is of high magnetic permeability; thus, it is easily saturated by a time-varying alternating current (A.C.) applied magnetic field. The saturated magnetic material yields non-linear response magnetic fields containing harmonic frequencies of the applied field frequency.
A problem with the known electronic surveillance systems arises due to the requirement that it interrogate a large space. Common magnetic objects, such as keys, differ from the magnetic markers in that they have a lower magnetic permeability. Thus, the common objects emit relatively fewer harmonic signals (at lower frequencies) than a high permeability object does. Therefore, to properly distinguish high permeability, soft magnetic material (the article marker) from low permeability, soft magnetic material (the house key), higher order harmonics must be sensed and processed by the electronic article surveillance system. However, a problem is that much less signal energy is inherently present in higher-order harmonics than in lower-order harmonics. Thus, the detection system necessarily tends to be relatively complex.
Additionally, to achieve a multiplicity of distinctly recognizable objects, a limited number of electronic article surveillance systems incorporate several discrete magnetic elements. Each element yields a slightly different response to the relatively uniform (spatially) interrogation and reading fields of the detector system. In this way, when a quasi-uniform interrogation field is applied to a tag or marker, the multiplicity of characteristics of the response magnetic field can be decoded to indicate tag identity. The separable characteristic can be identified as frequency, or as magnetic intensity switch-on threshold. No known attempt has been made in the prior art to gain spatially-resolved data from the anti-theft features by high resolution "reading" methods. This is because anti-theft applications require a detector coil of characteristic dimensions much larger than the size of the recognized feature (i.e., the tag).
Examples of prior art electronic article surveillance systems and its components are described and illustrated in EP 0295028, WO 88/09979, EP 0611164, EP 0352513, French Patent Specification 763681, and U.S. Pat. Nos. 3,665,449, 3,747,086, 3,790,945, 3,292,080, 4,074,249 and 5,005,001.
Accordingly, it is a primary object of the present invention to verify the authenticity and/or denomination of a. paper-based value document, such as currency or banknote paper, having an embedded security thread with magnetic features.
It is a general object of the present invention to interrogate the security thread with the magnetic field signal and to determine the authenticity and/or denomination of the paper from the magnetic response signal emitted from the thread.
It is another object of the present invention to provide a security thread with one or more regions of "soft" magnetic material, the thread typically being embedded entirely in a paper-based value document, and to provide a device that both verifies that the magnetic thread material is of a predetermined type and senses the spatial distribution of the magnetic material to determine a characteristic, such as the denomination, of the document.
It is another object of the present invention to provide a non-contact verifier device for sensing the type and distribution of magnetic material on a security thread utilizing an interrogating magnetic field.
It is yet another object of the present invention to impose an alternating current magnetic field from a non-contacting source onto a security thread coated with soft magnetic material in predetermined patterns, and to sense the magnetic field re-emitted by the security thread and determine, from the sensed field, one or more characteristics of a document in which the security thread is embedded.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.